This invention relates particularly to temperature control systems. One use for the invention is the provision of a soldering iron which operates at a relatively constant temperature.
It is well known that a piece of high permeability ferromagnetic material may be held at a relatively constant temperature near its effective Curie temperature if the radio frequency current of proper frequency and magnitude are passed through it. Such a device, however, maintains its temperature constant only over a very limited range of heat power (in watts) extracted from the load. Generally, a typical such device operates at temperatures above the effective Curie when the heat extracted by the load is very low, operates at or near the effective Curie over a very limited range in which heat in watts is extracted from the load, and its temperature falls off sharply as the heat in watts extracted by the load increases above said very limited range.
The very limited range referred to above may be extended by using a composite structure of a ferromagnetic layer clad upon a ccpper conductor as taught in U.S. Pat. No. 4,256,945, issued Mar. 17, 1981 to Philip S. Carter and John F. Krumme entitled: Alternating Current Electrically Resistive Heating Element Having Intrinsic Temperature Control.
In two later applications I have described such a composite device in which I observed that there was a voltage standing wave ratio (VSWR) in a composite heater of the type such as was described in the aforesaid U.S. Pat. No. 4,256,945. Two such applications are my U.S. patent applications Ser. No. 586,715 (filed Mar. 6, 1984) and Ser. No. 666,346 (filed Oct. 30, 1984) both entitled High Efficiency Autoregulating Heater.
A prior application of Clappier, Ser. No. 684,730 (filed Dec. 21, 1984), now U.S. Pat. No. 4,626,767, entitled Constant Current RF Generator, assigned to the same assignee as the present application, mentions the destructive effect on a power supply due to changes in the VSWR due to changes in the load impedance.
The application of William D. Hall, Ser. No. 749,637 (filed June 28, 1985), entitled Ferromagnetic Element With Temperature Regulation, is assigned to the same assignee as the present application. A continuation-in-part application, Ser. No. 003,288 was filed on Jan. 14, 1987, and the parent application abandoned. FIG. 6 of the parent application, teaches (1) means for maintaining a constant RF load current, and (2) a means for switching the RF power off when the temperature of the ferromagnetic heating elements is at, or close to, its Curie temperature. This is accomplished by means of a current sensor which develops an output voltage proportional to the current through the load. The current is higher than necessary to raise the temperatures of the load up to the effective Curie point of the material. The magnitude of voltage across the load decreases as the temperature rises in the neighborhood of the effective Curie Temperature due to the decrease of the load impedance magnitude. A signal is produced which is proportional to the difference between the load voltage and a reference voltage. This difference voltage activates a pulse generator circuit which shuts down the driver circuit 52 for a fixed period of time t.sub.o allowing the heater to cool. After t.sub.o, the driver turns on again, thus producing a pulsating RF current in the load.
In some applications, Hall's approach using a pulsed RF generator has a significantly higher electrical efficiency than the non-switching constant current mode of operation of resistance ferromagnetic heater described in said U.S. Pat. No. 4,256,945 patent. In the constant current mode, the heater of U.S. Pat. No. 4,256,945 when "idling" with a small thermal load near the Curie temperature, has a relatively low impedance and therefore presents a large impedance mismatch to the RF generator. This causes the generator to operate at a relatively low efficiency. In an application such as the hand soldering tool, the circuit is operating in this mode for extended periods. Hall's approach, when properly implemented, avoids operation into a badly mismatched load and consequently achieves higher efficiency.
My present invention includes an improvement upon the aforesaid Hall application.